1. Field of the Invention
The present invention relates to a light-emitting device and a method for manufacturing the light-emitting device. Further, the present invention relates to an electronic device on which the light-emitting device is mounted.
2. Description of the Related Art
In recent years, technological development has been remarkably made in the field of displays. In particular, the needs of the market have stimulated tremendous progress in the technology directed to increase in resolution of displays and thinning of displays.
In the next phase of this field, focus is placed on commercialization of a flexible display having a curved display area, and a variety of proposals have been made on manufacturing the flexible display (for example, Patent Document 1). A light-emitting device using a flexible substrate can be highly lightweight compared to the case of using a glass substrate or the like.
However, in commercialization of such a flexible display, the biggest problem is its short lifetime.
The lifetime of the flexible display is short because, for a substrate which should support a light-emitting element and protect the element from moisture, oxygen, or the like of the surroundings, a glass substrate that is not flexible cannot be used, and instead, a plastic substrate which has flexibility but high permeability and low heat resistance has to be used. Since the heat resistance of the plastic substrate is low, a protective film with high quality which needs a high-temperature process cannot be formed, and moisture entering through the plastic substrate has a great influence on the lifetime of the light-emitting element, furthermore, the light-emitting device. In Non-Patent Document 1, for example, an example in which a light-emitting element is formed over a substrate including polyethersulfone (PES) as a base and is sealed with an aluminum film to form a flexible light-emitting device is introduced; however, its lifetime is about 230 hours and the light-emitting device is miles away from commercialization. In Non-Patent Documents 2 and 3, an example of a flexible light-emitting device in which a light-emitting element is formed over a stainless steel substrate is introduced. In this flexible light-emitting device, moisture is prevented from entering through the stainless steel substrate; however, moisture cannot be prevented effectively from entering from the light-emitting element side. Therefore, as an attempt to improve the lifetime of the flexible light-emitting device, it is manufactured over the stainless steel substrate, and a sealing film obtained by repeatedly stacking plural kinds of materials is employed for the light-emitting element side.
Although a metal thin film such as an aluminum film or a stainless steel substrate has both flexibility and low permeability, it does not transmit visible light therethrough with a normal thickness. Thus, in the light-emitting device, a metal thin film or a stainless steel substrate is used for only one of a pair of substrates which sandwich a light-emitting element.